Deciphering Reaction Products in Formamidine-Based Perovskites with Methylammonium Chloride Additive
Liang Chen, Manman Hu, Seonghwan Lee, Jaehui Kim, Zhiying Zhao, Sun‐Phil Han, Myoung Soo Lah, Sang Il Seok
Abstract
The fabrication of formamidinium lead iodide (FAPbI 3 ) perovskite solar cells (PSCs) involves the addition of methylammonium chloride (MACl) to promote low-temperature α-phase formation and grain growth. However, as the added MACl deprotonates and volatilizes into methylamine (MA 0 ) and HCl for removal, MA 0 can chemically interact with formamidinium (FA + ), forming methyl formamidinium (MFA + ) as a byproduct. Despite its significance, the chemical interactions among FAPbI 3 perovskites, MACl additives, and their byproducts remain poorly understood. Our findings reveal that the FA + and MA 0 reaction primarily yields a mixture of cis / trans - N -methyl formamidinium iodide (MFAI) isomers, with cis -MFAI prevailing as the dominant species. Moreover, MFAI subsequently reacts with PbI 2 to yield fully formed cis -MFAPbI 3 2H-phase perovskite. We elucidated the effects of MFAI on the crystal growth, phase stability, and band gap of formamidine-based perovskites through the growth of single crystals. This research offers valuable insights into the role of these byproducts in influencing the efficiency and long-term stability of future PSCs.